A crossover between Chicago Fire and Chicago P.D. is not unheard of – characters go back and forth between the shows fairly often. However, a two-night crossover event inspired by the Oklahoma City bombing? That’s not something that happens every week.

“I knew it was going to be big when the man himself, Mr. Dick Wolf, came to see us to talk to us about it, and the first questions out of his mouth to us was, ‘Do you guys remember Oklahoma City?’ And everybody just froze,” Chicago P.D. star Sophia Bush said. “And he said, ‘As our country has gone through tragedies like that one, it’s the first responders who always set the tone and who always begin to put us back together, and I want to really examine that now that we’ve got these two shows with the fire fighters and the police.’ And we were like, ‘OK here we go, this is really going to be something intense and poignant.’”

Now that the crossover is ready to hit the small screen, Bush warns that it’s both “beautiful” and “stressful.” “It’s like a very taut wire,” she said. “The tension has everything just vibrating. It’s unlike anything I’ve seen on TV. It’s such a cool thing that they’re doing with our shows in the way that they overlap all the time, and this really takes that to the next level.”

But aside from the “high-threat situation,” Bush did say that her character, Erin Lindsay, would share a moment with Chicago Fire’s Kelly Severide. “We have a little situation together, he and I,” she teased of the sort-of lovebirds.

Looking ahead, Bush promised that the final two episodes of the season will give viewers more of Lindsay’s back story. And with that being said, we asked her just how far Lindsay would go to stand by Voight. “For her, what it always comes down to is her moral compass and what feels like the right thing in her heart and in her mind because Lindsay’s got a really strong sense of right and wrong, and it was really honed in the way that she grew up and it’s what has sort of given her almost that spidey sense,” she said. “She’s one of the only people who can tell Voight to shove it and does. And now I know some of the back story, which I’m sure we’ll discover eventually, about how she reacted in season 1 of Chicago Fire when he was having his whole situation with Lt. Casey. That set Lindsay off and I’m excited to, at some point, be able to tell that story. So she doesn’t just excuse Voight or go along with everything he says, but she also knows why he’s a great cop, because he’s willing to suffer the consequences if it means doing the right thing. And I think that’s why she trusts him in the way that she does.”